U.S. patent number 6,375,433 [Application Number 09/611,387] was granted by the patent office on 2002-04-23 for method and apparatus for controlling pump discharge pressure of a variable displacement hydraulic pump.
This patent grant is currently assigned to Caterpillar Inc.. Invention is credited to Hongliu Du, Noah D. Manring.
United States Patent |
6,375,433 |
Du , et al. |
April 23, 2002 |
Method and apparatus for controlling pump discharge pressure of a
variable displacement hydraulic pump
Abstract
A method and apparatus for controlling a pump discharge pressure
of a variable displacement hydraulic pump having a swashplate, and
a servo valve for controlling an angle of inclination of the
swashplate. The method and apparatus includes sensing a value of an
actual pump discharge pressure, determining a desired control
pressure using a first feedback linearization control law,
determining a desired servo valve spool position using a second
feedback linearization control law, and controlling the value of
the actual pump discharge pressure as a function of the first and
second feedback linearization control laws.
Inventors: |
Du; Hongliu (Dunlap, IL),
Manring; Noah D. (Chillicothe, IL) |
Assignee: |
Caterpillar Inc. (Peoria,
IL)
|
Family
ID: |
24448814 |
Appl.
No.: |
09/611,387 |
Filed: |
July 7, 2000 |
Current U.S.
Class: |
417/53; 417/212;
417/222.1 |
Current CPC
Class: |
F04B
1/324 (20130101); F04B 49/08 (20130101); F04B
2205/06 (20130101); F04B 2205/05 (20130101); F04B
2201/12051 (20130101) |
Current International
Class: |
F04B
49/08 (20060101); F04B 1/12 (20060101); F04B
1/32 (20060101); F04B 049/00 () |
Field of
Search: |
;417/53,212,217,218,222.2,222.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tyler; Cheryl J.
Attorney, Agent or Firm: Lundquist; Steve D.
Claims
What is claimed is:
1. A method for controlling a pump discharge pressure of a variable
displacement hydraulic pump having a swashplate, and a servo valve
for controlling an angle of inclination of the swashplate,
including the steps of:
sensing a value of an actual pump discharge pressure;
determining a desired control pressure using a first feedback
linearization control law;
determining a desired servo valve spool position using a second
feedback linearization control law; and
controlling the value of the actual pump discharge pressure as a
function of the first and second feedback linearization control
laws, wherein the first and second feedback linearization control
laws create a first order system response.
2. A method, as set forth in claim 1, further including the step of
modifying at least one of the first and second feedback
linearization control laws as a function of at least one adaptive
on-line learning algorithm.
3. A method, as set forth in claim 2, wherein the adaptive on-line
learning algorithm is adapted to monitor a parameter associated
with the pump.
4. A method, as set forth in claim 3, wherein the parameter is a
pressure carry-over angle of the swashplate as the pump hydraulic
pressure transitions from one of the discharge pressure and an
intake pressure to an other of the discharge pressure and the
intake pressure.
5. A method, as set forth in claim 1, further including the step of
incorporating a sliding mode control term in at least one of the
first and second feedback linearization control laws as a function
of unmodeled dynamics of the pump.
6. A method, as set forth in claim 1, wherein the first feedback
control law includes parameters associated with swashplate
dynamics, including the angle of inclination of the swashplate.
7. A method, as set forth in claim 6, wherein the first feedback
control law is adapted to function without parameters associated
with swashplate dynamics, including the angle of inclination of the
swashplate, as a function of the pump operating above a
predetermined pressure value.
8. An apparatus for controlling a pump discharge pressure of a
variable displacement hydraulic pump, comprising:
a swashplate inclinably mounted to the pump;
a servo valve hydraulically connected to the pump for controlling
an angle of inclination of the swashplate;
a pump discharge pressure sensor connected to an output port of the
pump; and
a controller electrically connected to the pump for sensing a value
of an actual pump discharge pressure, determining a desired control
pressure using a first feedback linearization control law,
determining a desired servo valve spool position using a second
feedback linearization control law, and controlling the value of
the actual pump discharge pressure as a function of the first and
second feedback linearization control laws, wherein the first and
second feedback linearization control laws create a first order
system response.
9. An apparatus, as set forth in claim 8, further including a
swashplate angle sensor connected to the swashplate.
10. An apparatus, as set forth in claim 8, further including a
control servo in contact with the swashplate and adapted to receive
pressurized fluid from the servo valve and responsively control the
angle of inclination of the swashplate.
11. An apparatus, as set forth in claim 8, further including a
control pressure sensor connected to the control servo for sensing
the pressure of the fluid.
12. An apparatus, as set forth in claim 8, wherein the controller
is further adapted for modifying at least one of the first and
second feedback linearization control laws as a function of at
least one adaptive on-line learning algorithm.
13. An apparatus, as set forth in claim 8, wherein the controller
is further adapted for incorporating a sliding mode control term in
at least one of the first and second feedback linearization control
laws as a function of unmodeled dynamics of the pump.
Description
TECHNICAL FIELD
This invention relates generally to a method and apparatus for
controlling a variable displacement hydraulic pump and, more
particularly, to a method and apparatus for controlling nonlinear
characteristics associated with the pump discharge pressure of a
variable displacement pump.
BACKGROUND ART
Variable displacement hydraulic pumps are used in a variety of
applications. For example, hydraulic construction machines,
earthworking machines, and the like, often use variable
displacement hydraulic pumps to provide the pressurized hydraulic
fluid flow required to perform desired work functions.
Operation of the pumps, however, is subject to variations in
pressure and flow output caused by variations in load requirements.
It has long been desired to maintain the pressure output of the
pumps in a consistent manner so that operation of the hydraulic
systems is well behaved and predictable. Therefore, attempts have
been made to monitor the pressure output of a pump, and control
pump operation accordingly to compensate for changes in
loading.
For example, U.S. Pat. Nos. 4,510,750 and 5,865,602, to Izumi et
al. and Nozari, respectively, disclose the use of feedback systems
which monitor characteristics such as pump output pressure, and
provide feedback control of the pump in attempts to operate the
pump in a desired manner. However, neither Izumi et al. nor Nozari
account for the wide range of nonlinearities inherent in hydraulic
pump operation. The disclosed patents of Izumi et al. and Nozari
are limited to a linear range of pump operation in which behavior
of the pump is fairly predictable and thus may be controlled using
well known linear control techniques.
Nonlinear control methods exist which may be used to control
systems having essentially nonlinear behavior, such as variable
displacement pumps. For example, one of the most common methods of
control is to first linearize a nonlinear system and then control
the resultant linear system. A common example of such a system
involves a Taylor Series linearization, which linearizes a small
portion of the system about an operating point, the portion being
essentially linear in nature to begin with. The drawback of a
method such as this is that predictable performance is only assured
if the system stays close to the particular point about which it is
linearized.
Another method is to use a technique commonly known as gain
scheduling, in which a series of operating points are selected,
then a small portion about each operating point is linearized,
e.g., by a method such as the Taylor Series. However, this results
in a discrete system which does not function well as the system
moves from one operating point to another.
A method known as feedback linearization may be used to transform
nonlinear dynamics of a system to linear equations, which may then
be used to control the system in an effective manner. For example,
in U.S. Pat. No. 5,666,806, Dietz discloses a system which uses
feedback linearization control laws to control the nonlinear
behavior of a hydraulic system, in particular the nonlinear
behavior of a hydraulic cylinder. However, the system disclosed by
Dietz incorporates nonlinearities from multiple sources, such as a
pump, cylinder, control valve, and the like. As a result, Dietz is
required to apply feedback linearization control laws to many
sources of nonlinearities, thus resulting in linearized equations
having multiple, i.e., fourth order, dynamic response
characteristics.
In the present invention, it is desired to control a single device,
i.e., a variable displacement hydraulic pump, within a hydraulic
system, and thus control the nonlinear characteristics associated
with the hydraulic pump. It is also desired to control the pump
using feedback linearization control laws to control the discharge
pressure of the pump over a wide range of nonlinear operating
conditions. Furthermore, it is desired to control the nonlinear
characteristics of the pump using feedback linearization control
laws which create a first order system tracking response, thus
providing control over nonlinearities without overshoot for step
response.
The present invention is directed to overcoming one or more of the
problems as set forth above.
DISCLOSURE OF THE INVENTION
In one aspect of the present invention a method for controlling a
pump discharge pressure of a variable displacement hydraulic pump
having a swashplate, and a servo valve for controlling an angle of
inclination of the swashplate, is disclosed. The method includes
the steps of sensing a value of an actual pump discharge pressure,
determining a desired control pressure using a first feedback
linearization control law, determining a desired servo valve spool
position using a second feedback linearization control law, and
controlling the value of the actual pump discharge pressure as a
function of the first and second feedback linearization control
laws.
In another aspect of the present invention an apparatus for
controlling a pump discharge pressure of a variable displacement
hydraulic pump is disclosed. The apparatus includes a swashplate
inclinably mounted to the pump, a servo valve hydraulically
connected to the pump for controlling an angle of inclination of
the swashplate, a pump discharge pressure sensor connected to an
output port of the pump, and a controller electrically connected to
the pump for sensing a value of an actual pump discharge pressure,
determining a desired control pressure using a first feedback
linearization control law, determining a desired servo valve spool
position using a second feedback linearization control law, and
controlling the value of the actual pump discharge pressure as a
function of the first and second feedback linearization control
laws.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic side profile cutaway view of a variable
displacement hydraulic pump suitable for use with the present
invention;
FIG. 2 is a diagrammatic end view of the pump of FIG. 1;
FIG. 3 is a diagrammatic illustration of a pump including a servo
valve;
FIG. 4 is a block diagram illustrating a preferred apparatus
including a control system for the pump of FIG. 3;
FIG. 5 is a feedback control diagram for the control system of FIG.
4;
FIG. 6 is a flow diagram illustrating a preferred method of the
present invention; and
FIG. 7 is a diagrammatic illustration depicting one aspect of the
present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to the drawings, a method and apparatus 100 for
controlling a pump discharge pressure of a variable displacement
hydraulic pump 102 is disclosed.
With particular reference to FIGS. 1 and 2, the variable
displacement hydraulic pump 102, hereinafter referred to as pump
102, is preferably an axial piston swashplate hydraulic pump 102
having a plurality of pistons 110, e.g., nine, located in a
circular array within a cylinder block 108. Preferably, the pistons
110 are spaced at equal intervals about a shaft 106, located at a
longitudinal center axis of the block 108. The cylinder block 108
is compressed tightly against a valve plate 202 by means of a
cylinder block spring 114. The valve plate includes an intake port
204 and a discharge port 206.
Each piston 110 is connected to a slipper 112, preferably by means
of a ball and socket joint 113. Each slipper 112 is maintained in
contact with a swashplate 104. The swashplate 104 is inclinably
mounted to the pump 102, the angle of inclination .alpha. being
controllably adjustable.
With continued reference to FIGS. 1 and 2, and with reference to
FIG. 3, operation of the pump 102 is illustrated. The cylinder
block 108 rotates at a constant angular velocity .omega.. As a
result, each piston 110 periodically passes over each of the intake
and discharge ports 204, 206 of the valve plate 202. The angle of
inclination .alpha. of the swashplate 104 causes the pistons 110 to
undergo an oscillatory displacement in and out of the cylinder
block 108, thus drawing hydraulic fluid into the intake port 204,
which is a low pressure port, and out of the discharge port 206,
which is a high pressure port.
In the preferred embodiment, the angle of inclination .alpha. of
the swashplate 104 inclines about a swashplate pivot point 316 and
is controlled by a servo valve 302. A servo valve spool 308 is
controllably moved in position within the servo valve 302 to
control hydraulic fluid flow at an output port 312 of the servo
valve 302. A discharge pressure feedback servo 304, in cooperation
with a servo spring 310, operates to increase the angle of
inclination .alpha. of the swashplate 104, thus increasing the
stroke of the pump 102. A control servo 306, receives pressurized
fluid from the output port 312 of the servo valve 302, and
responsively operates to decrease the angle of inclination .alpha.
of the swashplate 104, thus decreasing the stroke of the pump 102.
Preferably, the control servo 306 is larger in size and capacity
than the discharge pressure feedback servo 304. The pump 102
provides pressurized hydraulic fluid to the discharge port 206 of
the valve plate 202 by means of a pump output port 314.
Referring to FIG. 4, a block diagram illustrating a preferred
embodiment of the present invention is shown.
A pump discharge pressure sensor 404, preferably located at the
pump output port 314, is adapted to sense the output pressure of
the hydraulic fluid from the pump 102. Alternatively, the pump
output pressure sensor 404 may be located at any position suitable
for sensing the pressure of the fluid from the pump 102, such as at
the discharge port 206 of the valve plate 202, at a point along the
hydraulic fluid line from the pump 102 to the hydraulic system
being supplied with pressurized fluid, and the like. In the
preferred embodiment, the pump discharge pressure sensor 404 is of
a type well known in the art and suited for sensing pressure of
hydraulic fluid.
A control pressure sensor 408 is located at the control servo 306
in a manner suitable for sensing the pressure of the hydraulic
fluid being provided to the control servo 306 by the servo valve
302. Alternatively, the control pressure sensor 408 may be located
at the servo valve output port 312.
An optional swashplate angle sensor 406 is located at the
swashplate 104 in a manner suitable for sensing the angle of
inclination .alpha. of the swashplate 104. For example, the
swashplate angle sensor 406 may be a resolver mounted to the
swashplate 104, a strain gauge attached to the swashplate 104, or
some other type of sensor well known in the art. As discussed in
more detail below, the swashplate angle sensor 406 may not be
required with the present invention under certain
circumstances.
A controller 402, preferably located on a machine (not shown) which
uses the pump 102 as part of an overall hydraulic system, for
example a mobile construction or earthworking machine, and
electrically connected to the pump 102, is adapted to receive the
sensed information from the pump discharge pressure sensor 404, the
swashplate angle sensor 406, and any other sensors required, and
responsively perform a series of functions intended to control the
value of the hydraulic discharge pressure of the pump 102 in a
desired manner. More specifically, the controller 402 is adapted to
determine a desired pump discharge pressure using a first feedback
linearization control law, determine a desired servo valve spool
position using a second feedback linearization control law, and
control the value of the actual pump discharge pressure as a
function of the first and second feedback linearization control
laws. The operation of the controller is discussed in more detail
below.
Referring to FIG. 5, a feedback control diagram representative of a
preferred embodiment of the present invention is shown.
A desired pump discharge pressure P.sub.d is input into a first
junction 502, which also receives feedback from the output pressure
P of the pump.
The output of the first junction is provided to a first feedback
linearization control law 504 to determine a desired control
pressure P.sub.cd. Feedback linearization control laws, which in
theory are well known in the art, are used to transform a nonlinear
system to a global linear system.
In the preferred embodiment, the first feedback linearization
control law 504 is used for an outer loop 518 of the feedback
control system and may be represented by an exemplary equation of
the form: ##EQU1##
where
a.sub.c is the cross-section area of the control servo 306
multiplied by the distance from the control servo 306 to the
swashplate pivot point 316;
a.sub.p is the cross-section area of the discharge pressure
feedback servo 304 multiplied by the distance from the discharge
pressure feedback servo 304 to the swashplate pivot point 316,
which is added to a term representative of a swashplate pressure
carry-over angle .gamma. (which is described in more detail below
with reference to FIG. 7);
d is a spring bias term for the servo spring 310;
I(.alpha.).alpha.+G(.alpha..alpha.) are nonlinear dynamics of the
swashplate 104; and
.DELTA.P-k.sub.c.DELTA.P are error dynamics terms, where k.sub.c is
a gain constant which is greater than zero, and .DELTA.P represents
the difference between the actual discharge pressure of the pump
102 and the desired discharge pressure of the pump 102.
This input will result in a stable, convergent, first-order dynamic
output governed by the equation:
where, as Eq. 2 approaches zero, overshoot of the pump discharge
pressure P is eliminated.
It is noted that Eq. 1 is representative of an exemplary first
feedback linearization control law 504, and that variations of the
control law 504 may be used without deviating from the scope of the
present invention.
A second junction 506 receives the desired control pressure
P.sub.cd and also receives feedback from an inner loop 520. The
resultant output is then delivered to a second feedback
linearization control law 508, which is used to determine a desired
servo valve spool position x.sub.v. The second feedback
linearization control law 508 may be represented by an exemplary
equation of the form: ##EQU2##
where
C.sub.1c is a leakage coefficient of the control servo 306;
P.sub.c is the control pressure, i.e., pressure applied to the
control servo 306;
.alpha. is an angular velocity of the swashplate 104; ##EQU3##
is a capacitance of the control servo 306;
k.sub.x.DELTA.P.sub.c -P.sub.cd are control servo error dynamics
terms, where k.sub.x is a gain constant that is greater than
zero;
C.sub.d is a valve orifice coefficient for the,,servo valve spool;
and
w is the area rate which can be obtained by evaluating the
derivative of the area of the valve orifice at zero position.
Referring to the control servo error dynamics,
k.sub.x.DELTA.P.sub.c -P.sub.cd, .DELTA.P.sub.c =P.sub.c -P.sub.cd.
The resultant system error dynamics for the inner loop 520 are
given by:
where, as Eq. 4 approaches zero, overshoot of the control servo
control pressure P.sub.c is eliminated.
It is noted that Eq. 2 is representative of an exemplary second
feedback linearization control law 508, and that variations of the
control law 508 may be used without deviating from the scope of the
present invention.
The output from the second feedback linearization control law 508
is then delivered to a servo valve flow equation 510, to determine
the control pressure P.sub.c. Preferably, the servo valve flow
equation 510 is used to determine P.sub.c by first determining the
flow rate Q.sub.c controlled by the servo valve 302. An exemplary
equation to determine Q.sub.c is: ##EQU4##
where A.sub.o (x.sub..nu.) is the orifice area.
The control pressure P.sub.c is then compensated for various
swashplate dynamics 512, such as nonlinear friction on the
swashplate 104, Coulomb friction between each piston 110 and the
cylinder block 108, and the like. The output from the compensation
for the swashplate dynamics is then delivered to a third junction
514, in which the actual load flow rate Q.sub.L is combined. The
output from the third junction 514 is then compensated for hose
dynamics 516, such as compressibility of the hydraulic fluid,
leakage, and the like.
Referring to FIG. 6, a flow diagram illustrating a preferred method
of the present invention is shown.
In a first control block 602, the actual pump discharge pressure P
is sensed, preferably using the pump discharge pressure sensor 404
shown in FIG. 4. In addition, in the embodiment described with
relation to Eq. 1, the angle of inclination of the swashplate
.alpha. is determined, preferably by use of the swashplate angle
sensor 406 shown in FIG. 4.
However, in some high pressure applications, the terms associated
with the angle of inclination of the swashplate may be eliminated
without adversely affecting the use of the first feedback
linearization control law 504, i.e., Eq. 1. Therefore, the
swashplate angle sensor 406 is not needed in these applications. A
simplified first feedback linearization control law 504 may be
represented by: ##EQU5##
where ##EQU6##
In a second control block 604, the desired control pressure
P.sub.cd is determined using the first feedback linearization
control law 504, as explained above with reference to FIG. 5.
In a third control block 606, the desired servo valve spool
position x.sub..nu. is determined using the second feedback
linearization control law 508, as explained above with reference to
FIG. 5.
In an optional fourth control block 608, at least one of the first
and second feedback linearization control laws 504,508 is modified
as a function of at least one adaptive on-line learning algorithm.
An example of use of an adaptive on-line learning algorithm is
shown with reference to FIG. 7.
In FIG. 7, a valve plate 202 includes an intake port 204 and a
discharge port 206. The intake port 204 provides an intake pressure
P.sub.i, which is a low pressure, at a low pressure region 704. In
like manner, the discharge port 206 provides a discharge pressure
P.sub.d, which is a high pressure, at a high pressure region 702.
The transition area from the high pressure region 702 to the low
pressure region 704 is a pressure change region commonly known as a
swashplate pressure carry over angle .gamma.. As shown above with
respect to Eq. 1, the swashplate pressure carry over angle .gamma.
is included in the term a.sub.p, and thus has an effect on the
first feedback linearization control law 504. An adaptive on-line
learning algorithm may be used to compensate for the nonlinear
effects of .gamma.. An exemplary on-line learning algorithm may be
shown as:
and
where .phi. is a constant determined by factors such as the valve
plate geometry of the pump 102, the fluid bulk modulus, the nominal
volume of piston chambers in the cylinder block 108, the running
speed of the pump 102, and the like. As system conditions change,
these factors may change, thus causing changes in .gamma.. The
adaptive on-line learning algorithm, as a result, "learns" these
parameters under varying conditions, thus providing a stable,
convergent value for .gamma. in the first feedback linearization
control law 504.
In an optional fifth control block 610, a sliding mode control term
is incorporated into at least one of the first and second feedback
linearization control laws 504,508 as a function of bounded
unmodeled dynamics of the pump 102. Bounded unmodeled dynamics of
the pump 102 may include parameters which cannot be determined
mathematically, such as, but not limited to, temperature of the
hydraulic fluid, frictional forces, pressure errors, and the like.
An exemplary equation for sliding mode control is shown as:
##EQU7##
where indicates that the term is an estimated term, k.sub.s1 is a
constant that is greater than zero, s is a sliding surface term,
and .PHI. is the thickness of the boundary layer which determines a
performance bound for the system in sliding control. It is
understood that other sliding mode equations may be used without
deviating from the scope of the present invention.
In a sixth control block 612, the value of the actual pump
discharge pressure P is controlled as a function of the first and
second feedback linearization control laws 504,508.
INDUSTRIAL APPLICABILITY
As an example of operation of the present invention, a variable
displacement hydraulic pump 102 is often used to provide a supply
of pressurized hydraulic fluid to various actuators for performing
work functions. For example, work implements on earthworking
machines are typically powered by hydraulically actuated cylinders.
As the hydraulic actuators operate, various conditions create
nonlinearities in operation. For example, a work implement on an
earthworking machine commonly encounters rocks and other objects
which cause an increased demand in pressurized fluid from the pump
102.
It has long been desired to control the pressure of the fluid being
provided by a pump 102, but the nonlinearities imposed on the pump
make standard control techniques inefficient and unreliable.
The present invention is adapted to control the pressure being
delivered by a pump 102 by addressing the nonlinearities and
uncertainties associated with real life operations, i.e., by the
use of feedback linearization control laws and adaptive algorithms
which are targeted toward the actual nonlinear operation of a
variable displacement hydraulic pump 102.
Other aspects, objects, and features of the present invention can
be obtained from a study of the drawings, the disclosure, and the
appended claims.
* * * * *